The integrity of eukaryote genomes is derived in part from telomeres, the specialized nucleoprotein complexes that physically cap the chromosome terminus to distinguish it from a double-strand break to facilitate its complete replication. Loss of essential capping proteins or the telomere maintenance enzyme, telomerase, results in activation of DNA damage checkpoints and end-to-end chromosome fusions that culminate in massive genome instability. Defects in telomerase and/or telomere maintenance lead to proliferation-related human diseases, while correct telomerase regulation is essential to impede carcinogenesis. Elucidating how constituents of the telomere complex safeguard the genome is essential for understanding mechanisms that underlie human disease. Here we propose to exploit the many advantages of the Arabidopsis model system to investigate essential components of the telomere cap in a genetically tractable higher eukaryote. The prevailing view has been that telomeres are capped by two distinct complexes, shelterin in vertebrates and CST (Cdc13, Stn1, Ten1) in yeast. In the previous funding period, we discovered a new telomere complex in Arabidopsis comprised of STN1 and TEN1 orthologs and a novel telomere protein, CTC1. This new CST complex is required for telomere integrity in both plants and humans. The plant and human CTC1 orthologs share the same overall domain structure and non-specific ssDNA binding, distinct from yeast Cdc13. However, Arabidopsis CST plays a more pivotal role in chromosome end protection, akin to yeast CST. Thus, studies of Arabidopsis CST may provide a critical evolutionary bridge to elucidate mechanisms of telomere replication and chromosome end protection in higher eukaryotes. The central hypothesis of this renewal application that CTC1 is a multifunctional scaffold that protects chromosome ends and promotes telomeric DNA replication. Aim 1 will investigate interactions within the CST complex and explore the functional relevance of the TEN1- STN1 association for chromosome end protection. Aim 2 will assess the role of CST in telomere protection versus telomeric DNA replication. Aim 3 will test the hypothesis that POT1a recruits telomerase to the chromosome terminus via interactions with CTC1 and TER1. Finally, Aim 4 will employ forward genetic approaches and an innovative genetic screen possible only in Arabidopsis to isolate shelterin and CST- interacting factors and new telomere proteins. The studies will help to reconcile how CST and shelterin work together to promote telomere integrity. Altogether, the results generated from these four Aims will synergize to yield a much broader understanding of how telomeres stabilize higher eukaryotic genomes.